added check for reverse reaction being null
added check for reverse reaction being null

hopefully closes #52
The issue was probably precipitated by recent P-dep changes which made irreversible reactions possible

nyee/RMG-Java,connie/RMG-Java,KEHANG/RMG-Java,nyee/RMG-Java,jwallen/RMG-Java,nyee/RMG-Java,jwallen/RMG-Java,enochd/RMG-Java,ReactionMechanismGenerator/RMG-Java,keceli/RMG-Java,nyee/RMG-Java,keceli/RMG-Java,ReactionMechanismGenerator/RMG-Java,KEHANG/RMG-Java,rwest/RMG-Java,faribas/RMG-Java,ReactionMechanismGenerator/RMG-Java,connie/RMG-Java,rwest/RMG-Java,keceli/RMG-Java,connie/RMG-Java,ReactionMechanismGenerator/RMG-Java,ReactionMechanismGenerator/RMG-Java,connie/RMG-Java,KEHANG/RMG-Java,enochd/RMG-Java,faribas/RMG-Java,nyee/RMG-Java,KEHANG/RMG-Java,KEHANG/RMG-Java,rwest/RMG-Java,KEHANG/RMG-Java,jwallen/RMG-Java,faribas/RMG-Java,faribas/RMG-Java,keceli/RMG-Java,enochd/RMG-Java,enochd/RMG-Java,nyee/RMG-Java,enochd/RMG-Java,jwallen/RMG-Java,jwallen/RMG-Java,jwallen/RMG-Java,keceli/RMG-Java,connie/RMG-Java,rwest/RMG-Java,keceli/RMG-Java,enochd/RMG-Java,rwest/RMG-Java